Shallow trench isolation (STI) structures have been widely used to isolate areas within semiconductor devices. These STI structures are advantageous to miniaturization of semiconductor devices because a size of a field region is limited to a desired size of a trench by forming trenches in a semiconductor substrate and filling the trenches with insulation material.
Hereinafter, a conventional method of manufacturing the STI structure will be in brief described. FIGS. 1a to 1d are sectional views showing the conventional STI manufacture method.
First, as shown in FIG. 1a, a pad oxide film 2 is deposited at a thickness of about 200 Å on an entire surface of a silicon substrate 1. Subsequently, a silicon nitride film 3 is deposited at a thickness of about 2000 Å on the pad oxide film 2 and a photosensitive film is applied and exposed on the silicon nitride film 3. A pattern of photosensitive film 4 is then formed by removing only the photosensitive film on a region that is to include a trench.
Next, as shown in FIG. 1b, a trench 100 is formed in the silicon substrate 1 by dry etching the exposed silicon nitride film 3, the pad oxide film 2, and the silicon substrate 1 up to a predetermined depth using the pattern of photosensitive film 4 as a mask. The pattern of photosensitive film 4 is removed and then a cleaning process is performed.
As shown, an edge at which a side and a bottom of the formed trench intersect forms a nearly right angle. It is almost impossible to decrease this angle so that the edge is gently slanted.
Next, as shown in FIG. 1c, a liner oxide film 5 is formed at an inner wall of the trench 100 using a thermal oxidation process. According to one example, the liner oxide film 5 is formed at about 60% of its total thickness inside the silicon substrate 1 and at about 40% of the total thickness outside the silicon substrate 1 by a typical thermal oxidation process. The liner oxide file 5 is centered at a surface (shown as a dotted line in FIG. 1c) of the silicon substrate 1 of the trench.
During the thermal oxidation process for the formation of the liner oxide film 5, as an angle of an edge at which a side and a bottom of the trench 100 intersect becomes smaller, it becomes difficult for oxygen molecules to penetrate into the silicon substrate. For example, a nearly vertical trench edge creates a state in which oxygen molecules do not easily penetrate into the silicon substrate.
At this time, because most of deposition processes are performed using only heat in a high temperature without any electric power, the entire surface of the silicone substrate 1 assumes electrical neutrality. Under this state, as shown in FIG. 1d, a field oxide 6 is thickly deposited on an entire surface of the silicon nitride film 3 including the liner oxide film 5 such that the trench 100 is sufficiently buried or filled.
The field oxide 6 is consecutively deposited at the same speed on the silicon nitride film 3 or in the interior of the trench 100 on the liner oxide film 5 with a surface state as shown as a dotted line in FIG. 1d. During the deposition process, the field oxide 6 may create a shape that is difficult to fill. Accordingly, a void 200 in the field oxide 6 may be created in the interior of the trench 100. If this void 200 is excessively large, the void 200 will be exposed when a chemical mechanical polishing is performed to planarize the filed oxide 6. The exposing of the void 200 results in difficulties during the planarization process.
In addition, in a state where the void is exposed after the planarization, when a polysilicon to be deposited for formation of an electrode in a subsequent process enters the void, a leakage current may result, thereby causing erroneous operation of a device and a circuit-short between adjacent devices. These effects are fatal to the operation of the device.
The above problems become more serious as a width of the trench becomes narrower.
One prior approach to filling the trench without any void is disclosed in Korean Patent No. 36355, which discloses a technique by which a composite film structure of an anti-diffusion insulation film and a thermal oxide film is provided between a nitride film liner and a trench in order to minimize a transistor characteristic deterioration due to the nitride film liner. However, this approach has a disadvantage in that a manufacturing process for forming the composite film structure is complicated.
Another approach is disclosed in Korean Patent Application No. 2003-1409, which discloses techniques by which a first liner oxide film formed in an inner wall of a trench is etched away by a wet etching method, a second liner oxide film is thermally grown such that a top surface of the second liner oxide film has a smoothly curved edge, and then a filed oxide is formed on the top surface of the second liner oxide film such that the trench is completely filled without any void. However, because this technique requires a process of wet etching the liner oxide film and two deposition processes, this second prior approach also has a disadvantage of a complicated manufacturing process.
Still other prior approaches are disclosed in U.S. Pat. Nos. 6,531,413 and 6,214,698, which disclose techniques by which an undoped thin film is used for preventing voids, or a gap fill is achieved by two processes. However, since these techniques require a modification of process conditions, it also has a disadvantage of a complicated manufacturing process.